That's extremely serious because the call you're trying to make could be an emergency call. A bug like that would have top priority in the org I used to work in.
If I'd had to guess it cancels the call because there's a crash in a process somewhere. Possibly because of audio handover between apps.
I did a MS in Telecoms Engineering. Our Telephony teacher Claude Rigault drummed it into us that when people can't make emergency calls, pople die, thus the importance of reliability.
That's extremely serious because the call you're trying to make could be an emergency call. A bug like that would have top priority in the org I used to work in. If I'd had to guess it cancels the call because there's a crash in a process somewhere. Possibly because of audio handover between apps.